Make the selection set of the Select widget the input of the geoprocessing widget in Web AppBuilder

Summary

The Select widget, available in Web AppBuilder for Developers 2.1 and later, as well as the ArcGIS Online Web AppBuilder, provides the useful functionality to make a selection set and have that be the input of a geoprocessing service configured in the geoprocessing widget. However, to do this the underlying model or script must be configured in a way to accept 'user defined values'. The geoprocessing service must also accept a user defined value. 

Procedure

To have this functionality, the initial input into the model or script must be a transportable dataset.  A transportable dataset refers a data type that supports the user defined value input mode. 

The following are transportable datasets:

• Feature Class
• Feature Layer
• Feature Set
• Raster Layer
• Raster Dataset
• Table
• Table View
• Record Set

1. To determine what type of dataset a tool accepts, check the tool's reference page. 
2. Ensure that the initial input must also be made a model parameter, so that the user will be able to define the input. 

Once the model or script is configured, the service must be configured to accept user defined values. This is done in the service editor before publishing the service by selecting the parameter and Input mode: User defined value. 

At this point, the service can be published and configured in the geoprocessing widget, with the ability to accept values from the select widget.